Elsallia.com is the only real Langrisser resource in English I'm aware of. The forum is quiet, but interesting things still happen about once every few years. More importantly, ElSallia pretty much has everything you'll need to translate the games.

Translated, there is:
1. A translation of the PC port of the original Langrisser, which actually changed a lot and is more like the sequels. Highly recommended, although it's a bit of a pain to get working. You almost certainly need VirtualBox with an XP installation.
2. A translation of Langrisser 2 for genesis. Highly recommended. The only FAQ I ever wrote was on this game, over at GameFAQs.
3. A translation of Der Langrisser for the SNES. This game is a remake of Langrisser 2, only with a branching storyline and loads of other changes. In the end, it's actually quite different. They both have strengths and weaknesses. Highly recommended.
4. A translation of Langrisser IV for the PSX. This just came out, but you can find the patch on the forums under "New Projects." The Saturn release is rather superior to this, but it's still a great game. Highly recommended.

There is also a translation to the Crest of Gaia/Elthlead, but it's not nearly as good as the proper Langrisser games. I'd only recommend it as a curiosity. There's also a patch to revert the changes made to Langrisser when it was translated and became Warsong. However, there really aren't that many changes, so it's more of a curiosity than anything else.

Personally, if I were you, I'd probably try either Langrisser 2 or Der Langrisser. Langrisser 2 is a little tighter in its design and is far more challenging, but Der Langrisser gives you all kinds of choices and feels a lot bigger.

In general, the sequels kept the basic structure but added a few things here and there. The single biggest change is the introduction of true ranged units. While archers were just the paper to cavalry's rock in Warsong/Langrisser, they are actually ranged now. Ballistae are also introduced with -long- ranges and higher attack powers. Spearmen have replaced archers as the paper to cavalry's rock, which is fitting.

Other than that, the biggest shift is probably the ever-increasing introduction of RPG elements. Opening quizzes to see who you are, romantic interests, branching stories, more items, and more exploration - all seem, generally, to add RP elements to the series. Still, at the end of the day, this is for strategy gamers.